WASHINGTON — Nearly six months after President Barack Obama vowed to “bring to justice” the militants who killed the U.S. ambassador and three other Americans in Benghazi, Libya, the joint CIA and FBI effort to catch the ringleaders has made little apparent progress.

Officials say U.S. authorities do not yet have a full understanding of who planned and carried out the two brief but intense assaults, nearly eight hours apart, on a lightly guarded diplomatic compound and a nearby CIA base late on Sept. 11 and early the next day.

Investigators have identified several people who were present during the terrorist attacks, but none has been definitively linked to the deaths.